1. Field of the Invention
This invention relates to a wavelength multiplexed optical recording material permitting high density recording through the utilization of the phenomenon of photochemical hole burning.
2. Description of the Prior Art
At present, the optical disk has become most popular as the high density optical recording medium. Recording on the disk is performed by irradiating the disk with a recording laser beam which is narrowed to a diameter of the order of several microns, and the information is recorded in the form of changes in the optical concentration on the recording medium or in the form of changes in profile such as pit formation on the recording medium. Hence, only the information corresponding to one bit can be recorded at each laser spot, so that a limitation is imposed on increasing the recording density.
The overcome the limitation on the recording density, the concept of the so-called wavelength multiplexed recording has recently been introduced, according to which plural information units are recorded in each spot upon changing the wavelength of the recording laser. As one such system for wavelength multiplexed optical recording, a system employing a recording composition containing plural substances exhibiting photosensitivity to plural wavelengths has been reported for example in the Electronic Communication Association Technological Research Reports (CPM 82-55, pages 9 to 16). With this known system, the number of the information units that can be recorded in one spot is not larger than ten.
As a wavelength multiplexed optical recording system, separate from the above system, there is also known a method resorting to photochemical hole burning, the recording principle of which is described for example in the Japanese Laid-Open Patent Publication No. 53-99735 corresponding to the U.S. Pat. No. 4,101,976. According to this method, a broader absorption band the photosensitive material dispersed into the transparent medium exhibits at an ultra low temperature, hereafter referred to as the inhomogeneous absorption band, is irradiated with a laser beam of narrow bandwidth for forming sharply defined recesses, hereafter referred to as holes, in the absorption band. A large number of such holes can be formed in the inhomogeneous absorption band by changing the wavelength of the laser beam at small variations. Theoretically, it should be possible to form 10.sup.2 to 10.sup.3 holes in each inhomogeneous absorption band. This system is thought to be promising for high density recording since the number of the information units in each spot can be drastically increased through the utilization of the presence or absence of these holes for bit recording.
The photosensitive materials so far known to exhibit photochemical hole burning include porphine, chlorin, phthalocyanin, quinizarin or tetrazin, in addition to chromoproteins such as chlorophyle or phycocyanin.
It is noted that, in a photochemical reaction in general, the ratio of the number of the reacted molecules to that of the photons absorbed in the reaction system, that is, the so-called quantum yield, has been recognized as a critical factor. The material excellent in the quantum yield is advantageously employed for improving the recording sensitivity. This means that, when the light source of a predetermined light intensity is employed, the same effect may be achieved by the irradiation continuing for a shorter time interval, so that a high-speed recording may be achieved. However, it is thought that the photoisomerization reaction in the photochemical hole burning proceeds in many cases through the excited triplet state, this process being the rate-determining step. Therefore, it has been difficult to achieve a high speed in a number of materials thus far studied as the photosensitive material, so that it has not been possible to achive satisfactory recording sensitivity.
On the other hand, the degree of multiplexing in photochemical hole burning is determined by the number of independent holes having a hole width .DELTA.w.sub.h that can be formed in an inhomogeneous absorption band having a band width .DELTA.w.sub.i. Usually, the value of the ratio .DELTA.w.sub.i /2 .DELTA.w.sub.h is adopted as the measure of the degree of multiplexing. However, with the aforementioned known compounds, the multiplexing degree cannot be increased beyond a certain limit value because only larger hole widths can be attained with these compounds. In addition, in the known compounds, side holes may be produced to deteriorate the recording signals, while the saturation of the recording signals occurs at an earlier stage such that the S/N ratio cannot be elevated satisfactorily. Thus the known compounds cannot be employed for practical usage or application.